The blank well A1 does not contain any analytes or internal standards. The zero samples only contain internal standards, but no analytes. If there are still peaks the following may have occurred:

1. A sample or internal standard mix was accidentally loaded.
2. Spillover when loading the extraction solvent onto the kit plate, e.g. the dispensing speed of the repeater was too high.
3. Spillover when shaking the plate after loading the extraction solvent.
4. Cross-contamination by residues at the bottom of the filter plate when removing the filter plate from the capture plate.
5. Cross-contamination when transferring the extracts to make the LC and FIA plates.

• To make sure the contamination comes from the plate and not from the system, inject a blank from an autosampler vial to cross-check.